To reduce the size and cost of optical storage heads without sacrificing efficiency, considerable attention is being directed to optical and electronic integration. Optical integration seeks to reduce the number of components used for beam steering, polarization discrimination, circularization of the laser beam, and data and servo signal generation, and to eliminate or reduce precision alignment requirements for the individual components. Electronic integration aims to combine all opto-electronic components on a single module or chip.
U.S. Pat. No. 4,907,847 and various Japanese published unexamined patent applications describe prior attempts, employing low efficiency surface relief holograms, to reduce the number of components and hence overall cost of an optical storage system. For example, see JP1-13246, published Jan. 18, 1989; JP1-55745, published Mar. 2, 1989; and JP 63-25845, published Feb. 3, 1988.
All of the above patents describe optical storage heads incapable of magneto-optic recording except JP1-13246 which uses difficult to manufacture varying pitch gratings.
In surface relief holograms the holographic beam interference pattern may be recorded by a photolithographic process or using a mold and results in a periodic variation in the thickness of the holographic material without change of its refractive index. By contrast, in volume holograms, the interference pattern is recorded as a periodic variation in the refractive index of a holographic material whose thickness remains fixed. While processing volume holograms is usually more involved, they direct light much more efficiently than surface relief holograms. The angle of incidence at which maximum diffraction efficiency occurs is known as the Bragg angle.
Surface relief holograms are unable to achieve efficient polarization separation except for a limited range of diffraction angles. By contrast, volume holograms can be designed to achieve efficient beam separation for small angular changes and can achieve polarization separation over a wide range of angles, such as is taught in U.S. Pat. No. 4,497,534 issued Feb. 5, 1985. Polarization separation is especially important in a magneto-optical system wherein data is sensed as changes in the plane of polarization of the light reflected from the optical storage medium.
Commonly assigned copending application U.S. Ser. No. 07/774,410, for "Hologram System" filed Oct. 10, 1991 (Docket SA990033) (now abandoned) discloses a hologram system that employs a sandwich of two or four holograms to permit the small beam angle separations that are preferred in optical storage systems so that optical sensors may be located close to each other. However, this hologram sandwich generates only three beams (two data beams and one servo beam) and needs additional optical elements as a result of a lower degree of optical integration. Because only one servo beam is generated, the servo detector in this system requires many elements. Also, the laser and detector; are not integrated into a single monolithic array, and so the detectors require precision post-assembly alignment with the laser.
While the optical storage systems described above have to some degree reduced the complexity and number of components of an optical head, they do not achieve integration of the write path. Complete integration of the read, write and servo paths is essential for opto-electronic integration of all laser and detector devices on a single chip. To integrate on a single chip the laser, data and servo beams must have small angular separations and lie in a single plane to permit focusing on the PN-junction plane of the chip. The write path must maintain a single beam spot on the disk, yet allow multiple beams to be generated upon return and also compensate for dispersion effects associated with holograms and laser wavelength shifts. Also, polarization discrimination is necessary to allow the differential detection required for current state of the art magneto-optical recording media.
There is a need for an optical data storage system having a higher degree of both optical and electronic integration of an optical or magneto-optical storage head than is suggested by the prior art known to applicants.